1. Field of the Invention
This invention pertains to a metal casting system and more particularly, to the casting of metals employing molds within particulate beds subjected to vacuum conditions.
2. Description of the Prior Art
There are several foundry operations which provide for the casting of molten metal within molds held in a particulate bed under reduced pressure conditions. In one of these procedures, a rigid pattern is formed of a thermally degradable material, such as a porous plastic foam (e.g., styrofoam) which is tightly surrounded by the particles of a particulate bed, such as sand. The molten metal is introduced into the pattern, and thermally destroys same while filling the resultant cavity formed by the pattern within the particulate bed. Vaporization of the pattern produces large amounts of gases which are removed by applying a reduced pressure or vacuum to the particulate bed. The pattern is destroyed concomitantly with the filling thereof by the molten metal, and a new pattern must be prepared for each casting that is desired to be produced.
In another foundry operation, a pattern is provided having the shape of the desired casting. This pattern is then coated with a matrix material which can be hardened to provide a shell mold. The shell mold is rigid and self-sufficient so as to be introduced directly into the particulate bed where casting operations are to be performed. If desired, the pattern about which the shell mold is formed may be thermally destroyed before the shell mold is introduced into the bed. For this purpose, the pattern containing the matrix coating can be heated externally of the particulate bed. As a result, the pattern is vaporized leaving a hardened shell surrounding a cavity having the configuration of the pattern. In this method of casting, the shell mold is introduced into a particulate bed, usually fluidized, so that the shell mold may be positioned without injury. Then, the particulate bed is placed into a non-fluidized condition and packed firmly about the shell mold. Lastly, the molten metal is introduced into the cavity formed by the shell mold and produces the desired casting. Simultaneously with the pouring of the molten metal, it is usually desired that the particulate bed be aspirated or operated under reduced pressure conditions relative to the shell mold so that any resultant vapors, gases, etc., are withdrawn through the bed rather than having to be expelled against the flow of molten metal from the shell mold.
In either of the above casting procedures of the prior art, there results a metal casting of the desired shape surrounded by a particulate bed which includes the residual materials of the original mold. Usually, fluidization of the particulate bed will destroy such small amounts of residual materials. In this manner, the casting can be readily removed in a relatively clean condition from the bed.
In the several prior art metal casting procedures, a pattern is required which is consumed during the casting procedure, either directly during the receipt of the molten metal into the mold, or during some earlier step in mold fabrication before the actual casting of the metal occurs. Thus, there is a large waste of the material from which the pattern is formed, as well as energy-wasteful procedures which are necessary for removing the vaporized thermally degraded material produced by the destruction of the pattern.
In the present invention, a porous pattern is provided in the desired shape of the casting. This pattern is surrounded with a thin thermal degrading film in a tight-fitting relationship. Then, a particulate bed is packed tightly about the film-surrounded pattern. A vacuum on the particulate bed holds the film tightly against the bed. Now, the pattern is removed from the surrounding film leaving as a mold, a film defined cavity having the desired shape of the casting in the bed. The cavity can be filled conventionally with molten metal to produce the casting. As a result, only a small amount of film material is consumed for each casting produced by this novel system. The porous pattern can be reused as desired. The present system provides special advantages in the casting of cylindrical and tubular members in repetitive numbers at great savings in material and energy consumed during the casting procedure.